Iceyak,I design and make custom furniture and quite often find that I'm having to glue up mating surfaces that are round, tapered or just don't lend themselves to standard clamping procedures. One trick of the trade is to use sand bags. It will give you the weight needed as well as conform to the unusual shapes being glued together.

As for the glue... I haven't had any experience with gorilla but you'll definitely want to avoid something that is brittle once it has cured. You might want to consider rubber cement as it will flex along with the PVC tubing. It is best applied to both surfaces and waiting until they are tacky, then mating them. DO read the labels to be sure they can be exposed to water. One thing about using this method... get it right the first time or you may have to start over.